1. Field of the Invention
This invention relates to the field of measuring tension in a line, particularly for use on wirelines on drilling rigs.
2. Description of the Related Art
A drilling or workover rig as used in oil and gas exploration includes a derrick, draw works, surface equipment and a wireline used to moveably suspend a drill string from the rig. The wireline is usually a wire rope or steel cable, although other materials have also been used. The well drill string, even in a moderately deep well is highly flexibly and relatively to damage. Controlling the drill string weight applied to the drill bit is very important in drilling efficiency. The wireline at one end is wound on a drum or windless and is then passed in several turns between a crown block and a traveling block. The other end of the wireline is secured at the rig foundation or base. The traveling block carries a hook with a rotary connection to a kelly. The hook supports the weight of the drill string within the wellbore and as applied to the drill bit.
The wireline or cable is typically multistrand with multi-wires per strand. For example, a cable or wireline of one and one-eighth inch diameter can have a fiber core surrounded by eight strands each having nineteen steel wires. This wireline has a breaking strength of about forty tons but yet is pliable and wear resistant. Although the wirelines handle large loads, there is a finite load limit wherein breaking problems can occur. The tension conditions on the wireline in virtually every drilling rig is monitored. Various tension measuring devices have been employed to control the drill string weight on the drill bit and to indicate to the driller the tension conditions on the wireline. Typically, the wireline tension is measured using hydraulic tension transducers which are highly non-linear and temperature dependent. Modified strain gages are also used including a hydraulic sensor modified to accept a strain gage load cell in place of a hydraulic diaphragm which are very expensive to manufacture. Typical strain gauges are also susceptible to error due to temperature variations and mounted such that they are subject to physical harm due to contamination by the operating environment. Accordingly, a tension measuring device is needed which is simple to use and inexpensive to manufacture, and which also provides a linear output response signal which can be protected from the environment and temperature-compensated for temperature variation.
The system of the present invention including a tension measuring device mounted on a line, for example, a wireline on a drilling rig, wherein the tension measuring device is mounted between the wireline load and anchor ends. The tension measuring device includes an elongated body having two ends and a deflection block secured to each end of the elongated body for holding the wireline spaced apart from the elongated body. A clamping mechanism between the deflection blocks clamps the wireline to the elongated body wherein increased tension in the wireline will bend the elongated body. A pair strain gage elements are mounted inside of the elongated body for producing an output signal proportional to the bending of the elongated body. The strain gage element is, for example, positioned inside of the elongated body and is applied directly to the measure the tension and compression forces encountered during bending of the elongated body. The strain gauge bridge provides increased accuracy in calculating tension on a wire line by enabling calculation of tension by measurement of the tension and compression components. The strain gauge bridge also provides temperature compensation to enhance the accuracy of tension calcuations. The elongated body is hermetically sealed to protect the strain gauge from the elements. A linearizing signal device, for example, positioned within the elongated body, is connected to the strain gage element for processing the output signal and for providing a readable output representative of the tension on the wireline. The linearizing signal device provides for linearization of the output signal from the strain gage element and is, for example, a capacitive circuit or a linear signal processor. A wireline tension readout device is adapted to receive and display the readable output representative of the tension on the wireline.
Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.